In completing a product recovery well, such as in the oil and gas industry, several downhole tasks or functions must generally be performed with tools lowered through the well pipe or casing. These tools include, depending on the required tasks to be performed, perforating guns that ballistically produce holes in the well pipe wall to enable access to a target formation, setting tools that install well-sealing bridge plugs at a desired depth within the pipe, packer-setting tools that create a temporary seal about the tool and valves that are opened or closed.
Several tools can be configured as a single tool string lowered on the end of a long, hollow tube filled with fluid, the tools being hydraulically activatable in sequence by pressurizing either the fluid in the tube or the interior of the well casing. Such a configuration of tools has included perforating guns carefully arranged to perforate the well casing without damaging the internal hydraulic activation circuit of the string.
Within these tool strings, explosive-based firing heads are frequently employed to trigger the detonation of associated downhole perforating guns or other tools. Due to safety issues understood by those familiar with well completion techniques, the detonation of a gun by a firing head is typically accomplished by a series of linked detonations that effectively transfer a detonation from the firing head to the gun across separable joints in the tool string in an assembly that enables the armed firing head to be assembled to the gun at the last possible moment. The firing head is generally separated from the gun by a spacer of sufficient length that when the firing head is finally assembled to the tool string by workers on a rig platform, the associated gun, already lowered into the well casing, is safely below the platform. A detonating cord, sometimes called primacord, typically extends down an internal bore of the spacer between the firing head and its associated gun to link the firing head to the gun.